The argument isn't that it should be open sourced to improve spam detection or efficacy, the argument is that it's being misused as censorship mechanism without

The argument isn't that it should be open sourced to improve spam detection or efficacy, the argument is that it's being misused as censorship mechanism without calling it that, and that's why it's not open source - it would become readily apparent that it's being used for political censorship if it was open source. If it was just being used for spam filtering and not censorship, it would make much more sense to implement the blocking client-side (on the recipient's device) and provide a user-toggleable flag to disable that functionality. This would enable those who don't want spam and trust the filter to keep the exact same functionality, while allowing those who distrust it or suspect it of censorship to voluntarily opt-out for messages they receive. This would also offload processing costs from Signal's servers to user's devices, which would decrease the operational expenditure incurred by Signal. One reason not to take advantage of that economic incentive is that you don't want your users to have the choice, and you'd not want them to have the choice if you're trying to censor them.